Mobile communication system, user terminal, and base station

ABSTRACT

A mobile communication system according to an embodiment supports a cellular communication in which a data path passes through a network and a D2D communication that is a direct device-to-device communication in which a data path does not pass through the network. The mobile communication system comprises: an eNB  200  included in the network and configured to transmit D2D broadcast information; and a UE  100  configured to perform the D2D communication after receiving the D2D broadcast information from the eNB  200.  The D2D broadcast information is information that enables the D2D communication even though the UE  100  is in a specific state in which a connection with the network is not established. The UE  100  performs the D2D communication in the specific state on the basis of the D2D broadcast information.

TECHNICAL FIELD

The prevent invention relates to a mobile communication system, a userterminal, and a base station which support D2D communication.

BACKGROUND ART

In 3GPP (3rd Generation Partnership Project) which is a project aimingto standardize a mobile communication system, it is considered tointroduce communication between devices (Device to Device: D2D) as a newfunction to be specified in Release 12 or subsequent versions (see NonPatent Literature 1).

In the D2D communication, a plurality of neighboring user terminalsperform a direct communication without passing through a network. Thatis, a data path of the D2D communication does not pass through thenetwork. On the other hand, a data path of a normal communication(cellular communication) of a mobile communication system passes throughthe network.

CITATION LIST Non Patent Literature

[Non Patent Literature 1] 3GPP Technical Report “TR 22.803 V2.0.0”November 2012

SUMMARY OF THE INVENTION

The D2D communication is assumed to be controlled at the initiative ofthe network. Thus, a user terminal is considered to perform the D2Dcommunication in a state (a connected state) in which a connection withthe network has been established. However, such a method has a problemof an increase in load and signaling of the network caused by thecontrol of the D2D communication.

Therefore, the present invention provides a mobile communication systemcapable of suppressing an increase in load and signaling of a networkcaused by the control of D2D communication.

A mobile communication system according to the embodiment supportscellular communication in which a data path passes through a network andD2D communication that is direct device-to-device communication in whicha data path does not pass through the network. The mobile communicationsystem comprises: a base station that is included in the network andtransmits broadcast information; and a user terminal that performs theD2D communication after receiving the broadcast information from thebase station. The broadcast information is information that enables theD2D communication even though the user terminal is in a specific statein which a connection with the network is not established. The userterminal performs the D2D communication in the specific state on thebasis of the broadcast information.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a configuration diagram of an LTE system.

FIG. 2 is a block diagram of the UE.

FIG. 3 is a block diagram of the eNB.

FIG. 4 is a protocol stack diagram of a radio interface in the LTEsystem.

FIG. 5 is a configuration diagram of a radio frame used in the LTEsystem.

FIG. 6 is a diagram illustrating an operation environment according to afirst embodiment.

FIG. 7 is a diagram illustrating a D2D radio resource according to thefirst embodiment.

FIG. 8 is a diagram illustrating a specific example 1 of an interferenceavoidance operation pattern 2.

FIG. 9 is a diagram illustrating a specific example 2 of an interferenceavoidance operation pattern 2.

FIG. 10 is a diagram illustrating a specific example of a candidate of ahopping pattern.

DESCRIPTION OF EMBODIMENTS Overview of Embodiment

A mobile communication system according to a first embodiment and asecond embodiment supports a cellular communication in which a data pathpasses through a network, and a D2D communication that is a directdevice-to-device communication in which a data path does not passthrough the network. The mobile communication system includes a basestation included in the network and configured to transmit broadcastinformation, and a user terminal configured to receive the broadcastinformation from the base station and then performs the D2Dcommunication. The broadcast information is information that enables theD2D communication even in a specific state in which the user terminaldoes not establish a connection with the network. The user terminalperforms the D2D communication in the specific state on the basis of thebroadcast information.

In the first embodiment, the specific state is an idle state indicatinga state in which the user terminal does not establish the connection ina coverage of the network.

In the second embodiment, the specific state is a state in which theuser terminal exists out of the coverage of the network.

In the second embodiment, the base station is a base station thatmanages a termination cell included in a termination area of thecoverage.

In the first embodiment and the second embodiment, the broadcastinformation includes resource information indicating a radio resourcepermitted to be used in one of the D2D communication and a terminaldiscovery process for starting the D2D communication.

In the first embodiment and the second embodiment, the broadcastinformation includes power information indicating a maximum transmissionpower permitted in one of the D2D communication and a terminal discoveryprocess for starting the D2D communication.

In the first embodiment, the base station does not use the radioresource permitted to be used in one of the D2D communication and theterminal discovery process, in the cellular communication.

In the first embodiment, in the case of establishing the connectionbefore performing the D2D communication, the user terminal performs theD2D communication after disconnecting the connection on the basis of thebroadcast information.

In the first embodiment, in response to the detection of interference tothe D2D communication from another user terminal, the user terminalperforming the D2D communication transmits, to the network, informationindicating a request to avoid the interference after establishing theconnection or in the process of establishing the connection.

In the second embodiment, in response to the detection of interferenceto the D2D communication from another user terminal, the user terminalperforming the D2D communication determines to stop the D2Dcommunication and transmits information indicating the stop of the D2Dcommunication to a terminal with which the user terminal communicates.

In the first embodiment and the second embodiment, in response to thedetection of interference to the D2D communication from another userterminal, the user terminal performing the D2D communication performsnegotiation between terminals such that a radio resource used in a D2Dterminal group including the user terminal is different from a radioresource used in a D2D terminal group including the another userterminal.

In the first embodiment and the second embodiment, in response to thedetection of interference to the D2D communication from another userterminal, the user terminal performing the D2D communication changes aradio resource used in the D2D communication to another radio resource.

In the first embodiment and the second embodiment, the user terminal,which changes the radio resource used in the D2D communication to theanother radio resource, broadcasts change information indicating achange to the another radio resource by using the another radioresource.

In the second embodiment, when another user terminal, which belongs to aD2D terminal group different from the D2D terminal group including theuser terminal, receives the change information during the use of theanother radio resource, the another user terminal notifies a servingcell of the another user terminal of the reception of the changeinformation.

In the first embodiment and the second embodiment, when another userterminal, which belongs to a D2D terminal group different from the D2Dterminal group including the user terminal, receives the changeinformation during the use of the another radio resource, the anotheruser terminal notifies the user terminal of the fact that the anotherradio resource is being used.

In a modification of the second embodiment, the user terminal performsthe D2D communication by using a frequency hopping scheme. The broadcastinformation includes information indicating a hopping pattern permittedto be used in the D2D communication.

A user terminal according to the first embodiment and the secondembodiment is used in a mobile communication system that supports acellular communication in which a data path passes through a network,and a D2D communication that is a direct device-to-device communicationin which a data path does not pass through the network. The userterminal includes a receiver configured to receive broadcast informationfrom a base station included in the network, and a controller configuredto perform the D2D communication after the receiver receives thebroadcast information. The broadcast information is information thatenables the D2D communication even in a specific state in which the userterminal does not establish a connection with the network. Thecontroller performs the D2D communication in the specific state on thebasis of the broadcast information.

A base station according to the first embodiment and the secondembodiment is included in a network in a mobile communication systemthat supports a cellular communication in which a data path passesthrough a network, and a D2D communication that is a directdevice-to-device communication in which a data path does not passthrough the network. The base station includes a transmitter configuredto transmit broadcast information that enables the D2D communicationeven in a specific state in which a user terminal does not establish aconnection with the network.

First Embodiment

Hereinafter, with reference to the drawings, a description will beprovided for an embodiment in a case where D2D communication isintroduced to an LTE system which is one of mobile communication systemsconfigured based on the 3GPP standards.

(LTE System)

FIG. 1 is a configuration diagram of an LTE system according to thefirst embodiment. As illustrated in FIG. 1, the LTE system includes aplurality of UEs (User Equipment) 100, E-UTRAN (Evolved-UMTS TerrestrialRadio Access Network) 10, and EPC (Evolved Packet Core) 20. The E-UTRAN10 corresponds to a radio access network and the EPC 20 corresponds to acore network. The E-UTRAN 10 and the EPC 20 configure a network of theLTE system.

The UE 100 is a mobile communication device and performs radiocommunication with a cell (a serving cell) with which a connection isestablished. The UE 100 corresponds to the user terminal.

The E-UTRAN 10 includes a plurality of eNBs 200 (evolved Node-B). TheeNB 200 corresponds to a base station. The eNB 200 manages one or aplurality of cells and performs radio communication with the UE 100which establishes a connection with the cell of the eNB 200. It is notedthat the “cell” is used as a term indicating a minimum unit of a radiocommunication area, and is also used as a term indicating a function ofperforming radio communication with the UE 100.

The eNB 200, for example, has a radio resource management (RRM)function, a function of routing user data, and a measurement controlfunction for mobility control and scheduling.

The EPC 20 includes a plurality of MME (Mobility Management Entity)/S-GW(Serving-Gateway) 300. The MME is a network node for performing variousmobility controls and the like for the UE 100 and corresponds to acontroller. The S-GW is a network node that performs control to transferuser data and corresponds to a mobile switching center. The EPC 20including the MME/S-GW 300 accommodates the eNB 200.

The eNBs 200 are connected mutually via an X2 interface. Furthermore,the eNB 200 is connected to the MME/S-GW 300 via an S1 interface.

Next, the configurations of the UE 100 and the eNB 200 will bedescribed.

FIG. 2 is a block diagram of the UE 100. As illustrated in FIG. 2, theUE 100 includes an antenna 101, a radio transceiver 110, a userinterface 120, a GNSS (Global Navigation Satellite System) receiver 130,a battery 140, a memory 150, and a processor 160. The memory 150 and theprocessor 160 configure a controller. The UE 100 may not have the GNSSreceiver 130. Furthermore, the memory 150 may be integrally formed withthe processor 160, and this set (that is, a chip set) may be called aprocessor 160′.

The antenna 101 and the radio transceiver 110 are used to transmit andreceive a radio signal. The antenna 101 includes a plurality of antennaelements. The radio transceiver 110 converts a baseband signal outputfrom the processor 160 into the radio signal, and transmits the radiosignal from the antenna 101. Furthermore, the radio transceiver 110converts the radio signal received by the antenna 101 into the basebandsignal, and outputs the baseband signal to the processor 160.

The user interface 120 is an interface with a user carrying the UE 100,and includes, for example, a display, a microphone, a speaker, variousbuttons and the like. The user interface 120 receives an operation froma user and outputs a signal indicating the content of the operation tothe processor 160. The GNSS receiver 130 receives a GNSS signal in orderto obtain location information indicating a geographical location of theUE 100, and outputs the received signal to the processor 160. Thebattery 140 accumulates a power to be supplied to each block of the UE100.

The memory 150 stores a program to be executed by the processor 160 andinformation to be used for a process by the processor 160. The processor160 includes a baseband processor that performs modulation anddemodulation, encoding and decoding and the like on the baseband signal,and a CPU (Central Processing Unit) that performs various processes byexecuting the program stored in the memory 150. The processor 160 mayfurther include a codec that performs encoding and decoding on sound andvideo signals. The processor 160 executes various processes and variouscommunication protocols which will be described later.

FIG. 3 is a block diagram of the eNB 200. As illustrated in FIG. 3, theeNB 200 includes an antenna 201, a radio transceiver 210, a networkinterface 220, a memory 230, and a processor 240. The memory 230 and theprocessor 240 constitute a controller.

The antenna 201 and the radio transceiver 210 are used to transmit andreceive a radio signal. The antenna 201 includes a plurality of antennaelements. The radio transceiver 210 converts the baseband signal outputfrom the processor 240 into the radio signal, and transmits the radiosignal from the antenna 201. Furthermore, the radio transceiver 210converts the radio signal received by the antenna 201 into the basebandsignal, and outputs the baseband signal to the processor 240.

The network interface 220 is connected to the neighboring eNB 200 viathe X2 interface and is connected to the MME/S-GW 300 via the S1interface. The network interface 220 is used in communication performedon the X2 interface and communication performed on the S1 interface.

The memory 230 stores a program to be executed by the processor 240 andinformation to be used for a process by the processor 240. The processor240 includes the baseband processor that performs modulation anddemodulation, encoding and decoding and the like on the baseband signaland a CPU that performs various processes by executing the programstored in the memory 230. The processor 240 executes various processesand various communication protocols described later.

FIG. 4 is a protocol stack diagram of a radio interface in the LTEsystem. As illustrated in FIG. 4, the radio interface protocol isclassified into a layer 1 to a layer 3 of an OSI reference model,wherein the layer 1 is a physical (PHY) layer. The layer 2 includes aMAC (Media Access Control) layer, an RLC (Radio Link Control) layer, anda PDCP (Packet Data Convergence Protocol) layer. The layer 3 includes anRRC (Radio Resource Control) layer.

The PHY layer performs encoding and decoding, modulation anddemodulation, antenna mapping and demapping, and resource mapping anddemapping. Between the PHY layer of the UE 100 and the PHY layer of theeNB 200, data is transmitted via the physical channel.

The MAC layer performs priority control of data, and a retransmissionprocess and the like by hybrid ARQ (HARQ). Between the MAC layer of theUE 100 and the MAC layer of the eNB 200, data is transmitted via atransport channel. The MAC layer of the eNB 200 includes a transportformat of an uplink and a downlink (a transport block size and amodulation and coding scheme (MCS)) and a scheduler for determining aresource block to be assigned.

The RLC layer transmits data to an RLC layer of a reception side byusing the functions of the MAC layer and the PHY layer. Between the RLClayer of the UE 100 and the RLC layer of the eNB 200, data istransmitted via a logical channel.

The PDCP layer performs header compression and decompression, andencryption and decryption.

The RRC layer is defined only in a control plane. Between the RRC layerof the UE 100 and the RRC layer of the eNB 200, a control message (anRRC message) for various types of setting is transmitted. The RRC layercontrols the logical channel, the transport channel, and the physicalchannel in response to establishment, re-establishment, and release of aradio bearer. When there is an RRC connection between the RRC of the UE100 and the RRC of the eNB 200, the UE 100 is in a connected state (anRRC connected state), and when there is no RRC connection, the UE 100 isin an idle state (an RRC idle state).

A NAS (Non-Access Stratum) layer positioned above the RRC layer performssession management, mobility management and the like.

FIG. 5 is a configuration diagram of a radio frame used in the LTEsystem. In the LTE system, OFDMA (Orthogonal Frequency DivisionMultiplexing Access) is applied to a downlink, and SC-FDMA (SingleCarrier Frequency Division Multiple Access) is applied to an uplink,respectively.

As illustrated in FIG. 5, the radio frame is configured by 10 subframesarranged in a time direction, wherein each subframe is configured by twoslots arranged in the time direction. Each subframe has a length of 1 msand each slot has a length of 0.5 ms. Each subframe includes a pluralityof resource blocks (RBs) in a frequency direction, and a plurality ofsymbols in the time direction. The resource block includes a pluralityof subcarriers in the frequency direction.

Among radio resources assigned to the UE 100, a frequency resource canbe specified by a resource block and a time resource can be specified bya subframe (or slot).

In the downlink, an interval of several symbols at the head of eachsubframe is a control region used as a physical downlink control channel(PDCCH) for mainly transmitting a control signal. Furthermore, the otherinterval of each subframe is a region available as a physical downlinkshared channel (PDSCH) for mainly transmitting user data.

In the uplink, both ends in the frequency direction of each subframe arecontrol regions used as a physical uplink control channel (PUCCH) formainly transmitting a control signal. Further, the central portion inthe frequency direction of each subframe is a region mainly capable ofbeing used as a physical uplink shared channel (PUSCH) for transmittinguser data.

(D2D Communication)

The LTE system according to the first embodiment supports the D2Dcommunication that is direct communication between UEs. Hereinafter, theD2D communication will be described in comparison with normalcommunication (cellular communication) of the LTE system.

In the cellular communication, a data path passes through the EPC 20that is a core network. The data path indicates a communication path ofuser data (a user plane). On the other hand, in the D2D communication,the data path set between the UEs does not pass through the EPC 20.Thus, it is possible to reduce traffic load of the EPC 20.

The UE 100 discovers another UE 100 existing in the vicinity of the UE100 by a neighboring UE discovery (Discovery) process, and starts theD2D communication. The D2D communication, for example, is performed in afrequency band (a so-called licensed band) assigned to the LTE system.

The D2D communication includes a direct communication mode and a locallyrouted mode. In the direct communication mode, a data path does not passthrough the eNB 200. A UE group (a D2D UE group) including a pluralityof UEs 100 adjacent to one another directly perform radio communicationwith low transmission power in a cell of the eNB 200. Thus, a meritincluding reduction of power consumption of the UE 100 and decrease ofinterference to a neighboring cell can be obtained. On the other hand,in the locally routed mode, a data path passes through the eNB 200,however, not the EPC 20. The locally routed mode is able to reducetraffic load of the EPC 20, however, has a smaller merit as comparedwith the direct communication mode. Thus, in the first embodiment, thedirect communication mode is mainly assumed.

Operation According to First Embodiment

Next, an operation according to the first embodiment will be described.FIG. 6 is a diagram for describing an operation environment according tothe first embodiment. As shown in FIG. 6, UE 100-1D, UE 100-2D, and UE100-C exist in the cell of the eNB 200. In the first embodiment, the UE100-1D and the UE 100-2D perform D2D communication in the cell of theeNB 200. The UE 100-C performs cellular communication in the cell of theeNB 200. Hereinafter, a description will be provided for an operation inwhich the UE 100-1D and the UE 100-2D perform the D2D communication. Inaddition, hereinafter, the UE 100-1D and the UE 100-2D are simplywritten as “UE 100-D” when they are not particularly distinguished fromeach other.

Firstly, the eNB 200 secures a radio resource (hereinafter, a “D2D radioresource”) permitted to be used in the D2D communication. The D2D radioresource is designated by a time resource and/or a frequency resource.The time resource, for example, is a subframe. The frequency resource,for example, is a resource block and/or a frequency band. In the firstembodiment, the D2D radio resource is a dedicated radio resource that isnot commonly used together with a cellular radio resource for thecellular communication. FIG. 7 is a diagram illustrating a D2D radioresource according to the first embodiment. As shown in FIG. 7, amongradio resources corresponding to three subframes, several resourceblocks positioned at the center in the central subframe are secured asthe D2D radio resource. That is, the eNB 200 does not use the D2D radioresource in the cellular communication.

Secondly, the eNB 200 transmits broadcast information (hereinafter, D2Dbroadcast information) that enables the D2D communication even in aspecific state in which the UE 100-D does not establish an RRCconnection with the network. In the first embodiment, the specific stateis an idle state indicating a state in which the UE 100-D does notestablish the RRC connection in a coverage of the network. The eNB 200may periodically transmit the D2D broadcast information, or may transmitthe D2D broadcast information when detecting a predetermined trigger.The D2D broadcast information may be included in a system informationblock (SIB) or a master information block (MIB). The SIB or the MIB isinformation receivable in UE 100 in an idle state. The D2D broadcastinformation includes resource information indicating the D2D radioresource and power information indicating maximum transmission powerpermitted in the D2D communication. The D2D broadcast information mayalso include information on a signal transmitted and received in theDiscovery process (details will be described later).

Thirdly, the UE 100-D in a connected state or an idle state in the cellof the eNB 200 receives the D2D broadcast information from the eNB 200,and acquires the resource information and the power information includedin the D2D broadcast information. The UE 100-D may receive the D2Dbroadcast information before the Discovery process, or receive the D2Dbroadcast information after the Discovery process.

Fourthly, the UE 100-D in an idle state starts the D2D communication onthe basis of the D2D broadcast information. When the UE 100-D is in aconnected state before performing the D2D communication, the UE 100-Ddisconnects the RRC connection and then performs the D2D communicationin an idle state according to an instruction from the eNB 200 orvoluntarily. The UE 100-D decides a radio resource to be used in the D2Dcommunication from among D2D radio resources indicated by the resourceinformation, and performs the D2D communication by using the decidedradio resource. Furthermore, the UE 100-D decides transmission power tobe used in the D2D communication in a range of maximum transmissionpower indicated by the power information, and performs the D2Dcommunication by using the decided transmission power.

As described above, the UE 100-D performs the D2D communication in anidle state, so that it is possible to suppress an increase in load andsignaling of the network caused by the control of the D2D communication.

However, during the D2D communication, the UE 100-D may receiveinterference from UE 100-X (a cellular UE or a D2D UE belonging toanother D2D UE group) with which the UE 100-D does not communicate.Hereinafter, a description will be provided for operation patterns 1 to3 for avoiding interference during the D2D communication.

(1) Interference Avoidance Operation Pattern 1

The UE 100-D performing the D2D communication detects interference(interference power) to the D2D communication from the UE 100-X withwhich the UE 100-D does not communicate. When the interference isdetected, the UE 100-D transmits, to the eNB 200, information indicatinga request to avoid the interference after establishing the RRCconnection or in the process of establishing the RRC connection. Thatis, the UE 100-D transitions to a connected state and requests the eNB200 to perform a process for avoiding the interference. In the case ofestablishing the RRC connection only in order to request the process foravoiding the interference, the UE 100-D may notify the eNB 200 to thateffect during the process for establishing the RRC connection. As theprocess for avoiding the interference, the eNB 200 allows a radioresource used by the UE 100-X to be different from a radio resource usedby the UE 100-D, for example. Alternatively, the eNB 200 reduces thetransmission power of the UE 100-X.

(2) Interference Avoidance Operation Pattern 2

The UE 100-D performing the D2D communication detects interference(interference power) to the D2D communication from the UE 100-X withwhich the UE 100-D does not communicate. When the interference isdetected, the UE 100-D performs negotiation between UEs in order toavoid the interference while maintaining an idle state. For example, theUE 100-D negotiates with the UE 100-X such that a radio resource used bya D2D UE group including the UE 100-D is different from a radio resourceused by a D2D UE group including the UE 100-X.

(3) Interference Avoidance Operation Pattern 3

The UE 100-D, which performs the D2D communication by using a radioresource (hereinafter, a “radio resource A”) included in the D2D radioresource, detects interference (interference power) to the D2Dcommunication from the UE 100-X with which the UE 100-D does notcommunicate. When the interference is detected, the UE 100-D changes aradio resource used in the D2D communication to another radio resource(hereinafter, a “radio resource B”) included in the D2D radio resource.Then, the UE 100-D broadcasts change information indicating a change tothe radio resource B by using the radio resource B.

In this case, when UE 100-Y using the radio resource B receives thechange information, the UE 100-Y notifies a serving cell (the eNB 200)of the UE 100-Y of the reception of the change information. Furthermore,the UE 100-Y broadcasts in-use information, which indicates that theradio resource B is being used, by using the radio resource B. When thein-use information is received from the UE 100-Y, the UE 100-D performsone of the following processes.

-   -   The UE 100-D stops a change to the radio resource B when the        interference from the UE 100-X is reduced, and uses the radio        resource A.    -   The UE 100-D performs the process of the interference avoidance        operation pattern 1 or 2 when the interference from the UE 100-X        is not reduced.

Meanwhile, when the in-use information is not received from the UE100-Y, the UE 100-D notifies UE with which the UE 100-D communicates, ofa change to the radio resource B. Then, the UE 100-D and the UE withwhich the UE 100-D communicates perform the D2D communication by usingthe radio resource B.

FIG. 8 is a diagram illustrating a specific example 1 of theinterference avoidance operation pattern 2. In FIG. 8, a cell A is acell belonging to a frequency band A included in the D2D radio resource,and a cell B is a cell belonging to a frequency band B included in theD2D radio resource.

As shown in FIG. 8(A), UE 100-1 and UE 100-2 constitute a D2D UE group,and UE 100-3 and UE 100-4 constitute another D2D UE group. These two D2DUE groups are adjacent to each other and use the same frequency band,resulting in the occurrence of interference between the D2Dcommunications. Hereinafter, the case, in which the UE 100-4 detectsinterference from the UE 100-1, is considered.

As shown in FIG. 8(B), when interference is detected, the UE 100-4changes a frequency band (a cell), in which D2D communication isperformed, from the frequency band A (the cell A) to the frequency bandB (the cell B). Then, the UE 100-D broadcasts change information, whichindicates a change to the frequency band B (the cell B), in thefrequency band B (the cell B).

As shown in FIG. 8(C), since no in-use information is received, the UE100-4 notifies the UE 100-3 of a change to the frequency band B (thecell B). Then, the UE 100-3 and the UE 100-4 perform the D2Dcommunication in the frequency band B (the cell B).

FIG. 9 is a diagram illustrating a specific example 2 of theinterference avoidance operation pattern 2. Hereinafter, the differencerelative to the specific example 1 will be described.

As shown in FIG. 9(A), the UE 100-4 detects interference from the UE100-1 in the frequency band A (the cell A). Meanwhile, the UE 100-3 andthe UE 100-4 perform the D2D communication in the frequency band B (thecell B).

As shown in FIG. 9 (B), the UE 100-4 broadcasts change information,which indicates a change to the frequency band B (the cell B), in thefrequency band B (the cell B). UE 100-6 receives the change informationfrom the UE 100-4, and broadcasts (or notifies) in-use information,which indicates that the frequency band B (the cell B) is being used, inthe frequency band B (the cell B). The UE 100-4 receives the in-useinformation from the UE 100-6.

As shown in FIG. 9(C), since the interference from the UE 100-2 is notreduced, the UE 100-4 which has received the in-use information performsthe process of the interference avoidance operation pattern 1 or 2.

Second Embodiment

Next, a second embodiment will be described. In the second embodiment,the specific state is a state (hereinafter, an “out-of-service state”)in which the UE 100-D exists out of a coverage of the network. Thecoverage is not limited to a coverage of an LTE network, and may becoverages of all networks operated by the same communication provider.Furthermore, out-of-coverage indicates both an area in which radio wavesfrom the network do not reach, and an area in which the radio waves fromthe network are severely weak. An area out of the coverage is called an“out-of-range area”.

In the second embodiment, the eNB 200, which transmits D2D broadcastinformation, manages a termination cell included in a termination areaof the coverage. In the termination cell, at least a part of a peripherythereof is the out-of-range area. That is, the eNB 200 transmits the D2Dbroadcast information in the termination cell. The eNB 200 may benotified of information regarding whether the cell of the eNB 200 is thetermination cell from the EPC 20.

Hereinafter, an operation according to the second embodiment will beexplained while focusing on the difference relative to the firstembodiment.

The eNB 200 transmits D2D broadcast information that enables D2Dcommunication even though the UE 100-D is in the out-of-service state.The D2D broadcast information includes information (termination cellinformation) indicating that the D2D communication is permitted in theout-of-range area, in addition to resource information indicating a D2Dradio resource and power information indicating maximum transmissionpower permitted in the D2D communication.

The UE 100-D in a connected state or an idle state in the cell of theeNB 200 receives the D2D broadcast information from the eNB 200. Then,the UE 100-D transitioned to the out-of-service state performs aDiscovery process on the basis of the D2D broadcast information, andthen performs the D2D communication.

As described above, the UE 100-D performs the D2D communication in theout-of-service state, so that it is possible to effectively utilize theD2D communication and to enable communication even in the out-of-servicestate.

Furthermore, in the second embodiment, among the interference avoidanceoperations according to the first embodiment, an interference avoidanceoperation, other than the operation (that is, the interference avoidanceoperation pattern 1) to request the eNB 200 to avoid the interference,is applicable.

Moreover, in the second embodiment, instead of the aforementionedinterference avoidance operation pattern 1, an operation for stoppingthe D2D communication may be performed. In this case, in response to thedetection of interference to the D2D communication from the UE 100-X,the UE 100-D determines to stop the D2D communication and transmitsinformation indicating the stop of the D2D communication to acommunication destination.

Modification of Second Embodiment

In the second embodiment, since it is difficult to request the eNB 200to avoid the interference, D2D communication may be performed using afrequency hopping scheme in order to attenuate the influence ofinterference.

In the present modification, the D2D broadcast information includesinformation on a hopping pattern (candidates of the hopping pattern)permitted to be used in the D2D communication. However, the UE 100-D mayhold the candidates of the hopping pattern in advance. FIG. 10 is adiagram illustrating a specific example of the candidates of the hoppingpattern. In FIG. 10, a horizontal axis denotes a time axis and indicates10 subframes corresponding to one radio frame. A vertical axis denotes afrequency axis and indicates a bandwidth corresponding to six resourceblocks.

The UE 100-D selects a hopping pattern to be used in the D2Dcommunication from the candidates of the hopping pattern, and notifies acommunication destination of the selected hopping pattern. The UE 100-Dperforms the D2D communication by using the selected hopping pattern.

When the UE 100-D detects interference from the UE 100-X using the samehopping pattern, the UE 100-D may decide the right of use of the hoppingpattern by negotiation between UEs. When it is not possible to use theselected hopping pattern, the UE 100-D reselects another hopping patternfrom the candidates of the hopping pattern, and notifies thecommunication destination of the selected hopping pattern. The UE 100-Dperforms the D2D communication by using the reselected hopping pattern.

In addition, as well as the case of selecting (or reselecting) a hoppingpattern from the candidates of the hopping pattern, the corresponding UEmay hold a UE-specific hopping pattern and perform the D2D communicationby using the UE-specific hopping pattern. The hopping pattern may becalculated from a UE-specific ID, an ID of a cell in which the UEexists, a temporary ID (C-RNTI) assigned from the corresponding cell tothe UE, and so on.

Other Embodiments

In each of the aforementioned embodiments, the D2D broadcast informationmay include information on a signal (Discovery signal) transmitted andreceived in the Discovery process. The Discovery signal is a signal fordiscovering neighboring UE or a signal for being discovered by theneighboring UE. The information on the Discovery signal includesresource information indicating radio resources (Discovery radioresources) permitted to be used in the Discovery process, and powerinformation indicating maximum transmission power (Discovery maximumtransmission power) permitted in the Discovery process. In this case,the UE 100-D decides a radio resource used in the transmission of theDiscovery signal from the Discovery radio resources indicated by theresource information, and transmits the Discovery signal by using thedecided radio resource. Furthermore, the UE 100-D decides transmissionpower of the Discovery signal in a range of the Discovery maximumtransmission power indicated by the power information, and transmits theDiscovery signal by using the decided transmission power.

Each the embodiments and the modification mentioned above may beperformed separately and independently and may also be performed througha combination thereof.

In the aforementioned second embodiment and the modification thereof, aparameter (a radio resource, maximum transmission power and so on)necessary for the D2D communication may be statically decided and the UE100-D may hold information (the parameter) thereof. In this case, the UE100-D can perform the D2D communication by using the held parameterregardless of the D2D broadcast information (and the termination cellinformation).

Each of the aforementioned embodiments has described an example in whichthe present invention is applied to the LTE system. However, the presentinvention may also be applied to systems other than the LTE system, aswell as the LTE system.

Thus, the present invention includes a variety of embodiments notdescribed herein as a matter of course. Further, it is possible tocombine embodiments and modifications described above. Therefore, thetechnical scope of the present invention is defined only by the mattersaccording to claims based on the above description.

The entire contents of U.S. Provisional Application No. 61/766548 (filedon Feb. 19, 2013) are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide a mobilecommunication system, a user terminal, and a base station capable ofsuppressing an increase in load and signaling of a network caused by thecontrol of D2D communication.

1. A mobile communication system, which supports a cellularcommunication in which a data path passes through a network and a D2Dcommunication that is a direct device-to-device communication in which adata path does not pass through the network, comprising: a base stationincluded in the network and configured to transmit broadcastinformation; and a user terminal configured to perform the D2Dcommunication after receiving the broadcast information from the basestation, wherein the broadcast information is information that enablesthe D2D communication even though the user terminal is in a specificstate in which a connection with the network is not established, and theuser terminal performs the D2D communication in the specific state onthe basis of the broadcast information.
 2. The mobile communicationsystem according to claim 1, wherein the specific state is an idle stateindicating a state in which the user terminal does not establish theconnection in a coverage of the network.
 3. The mobile communicationsystem according to claim 1, wherein the specific state is a state inwhich the user terminal exists out of a coverage of the network.
 4. Themobile communication system according to claim 3, wherein the basestation is a base station that manages a termination cell included in atermination area of the coverage.
 5. The mobile communication systemaccording to claim 1, wherein the broadcast information includesresource information indicating a radio resource permitted to be used inone of the D2D communication and a terminal discovery process forstarting the D2D communication.
 6. The mobile communication systemaccording to claim 1, wherein the broadcast information includes powerinformation indicating a maximum transmission power permitted in one ofthe D2D communication and a terminal discovery process for starting theD2D communication.
 7. The mobile communication system according to claim5, wherein the base station does not use the radio resource permitted tobe used in one of the D2D communication and the terminal discoveryprocess, in the cellular communication.
 8. The mobile communicationsystem according to claim 1, wherein the user terminal performs the D2Dcommunication after disconnecting the connection on the basis of thebroadcast information, when the connection is established before the D2Dcommunication is performed.
 9. The mobile communication system accordingto claim 2, wherein the user terminal performing the D2D communicationtransmits, to the network, information indicating a request to avoid aninterference after establishing the connection or in a process ofestablishing the connection, in response to detection of theinterference to the D2D communication from another user terminal. 10.The mobile communication system according to claim 3, wherein the userterminal performing the D2D communication determines to stop the D2Dcommunication and transmits information indicating stop of the D2Dcommunication to a terminal with which the user terminal communicates,in response to detection of an interference to the D2D communicationfrom another user terminal.
 11. The mobile communication systemaccording to claim 1, wherein the user terminal performing the D2Dcommunication performs negotiation between terminals such that a radioresource used in a D2D terminal group including the user terminal isdifferent from a radio resource used in another D2D terminal groupincluding another user terminal, in response to detection of aninterference to the D2D communication from the another user terminal.12. The mobile communication system according to claim 1, wherein theuser terminal performing the D2D communication changes a radio resourceused in the D2D communication to another radio resource, in response todetection of an interference to the D2D communication from another userterminal.
 13. The mobile communication system according to claim 12,wherein the user terminal, which changes the radio resource used in theD2D communication to the another radio resource, broadcasts changeinformation indicating a change to the another radio resource by usingthe another radio resource.
 14. The mobile communication systemaccording to claim 13, wherein the another user terminal notifies a theanother user terminal's serving cell of a fact that the changeinformation is received, when the another user terminal, which belongsto a D2D terminal group different from a D2D terminal group includingthe user terminal, receives the change information during using theanother radio resource.
 15. The mobile communication system according toclaim 13, wherein the another user terminal notifies the user terminalof a fact that the another radio resource is being used, when theanother user terminal, which belongs to a D2D terminal group differentfrom a D2D terminal group including the user terminal, receives thechange information during using the another radio resource,
 16. Themobile communication system according to claim 1, wherein the userterminal performs the D2D communication by using a frequency hoppingscheme, and the broadcast information includes information indicating ahopping pattern permitted to be used in the D2D communication.
 17. Auser terminal, which is used in a mobile communication system thatsupports a cellular communication in which a data path passes through anetwork and a D2D communication that is a direct device-to-devicecommunication in which a data path does not pass through the network,comprising: a receiver configured to receive broadcast information froma base station included in the network; and a controller configured toperform the D2D communication after the receiver receives the broadcastinformation, wherein the broadcast information is information thatenables the D2D communication even though the user terminal is in aspecific state in which a connection with the network is notestablished, and the controller performs the D2D communication in thespecific state on the basis of the broadcast information.
 18. A basestation, which is included in a network in a mobile communication systemthat supports a cellular communication in which a data path passesthrough the network, and a D2D communication that is a directdevice-to-device communication in which a data path does not passthrough the network, comprising: a transmitter configured to transmitbroadcast information that enables the D2D communication even though auser terminal is in a specific state in which a connection with thenetwork is not established.